(define (eval exp env)
  (cond ((self-evaluating? exp) exp)
        ((variable? exp) (lookup-variable-value exp env))
        ((quoted? exp) (text-of-quotation exp))
        ((assignment? exp) (eval-assignment exp env))
        ((definition? exp) (eval-definition exp env))
        ((if? exp) (eval-if exp env))
        ((lambda? exp)
         (make-procedure (lambda-parameters exp)
                         (lambda-body exp)
                         env))
        ((begin? exp) 
         (eval-sequence (begin-actions exp) env))
        ((cond? exp) (eval (cond->if exp) env))
        ((application? exp)
         (apply (eval (operator exp) env)
                (list-of-values (operands exp) env)))
        (else
         (error "Unknown expression type -- EVAL" exp))))

• http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-26.html

We want to write a function that generates accumulators-- a function that takes a number n, and returns a function that takes another number i and returns n incremented by i.

JavaScript

function foo(n) { return function (i) { return n += i } }

Scheme

(define (foo n) (lambda (i) (set! n (+ n i)) n))

Try

JavaScript

function main() { function foo (n) { return function (i) { return n += i } } ; a = foo(5); print(a(1)); print("n"); print(a(4)); }

Scheme

(define (foo n) (lambda (i) (set! n (+ n i)) n)) (define a (foo 5)) (a 1) (a 4)

• via Paul Graham's Revenge of the Nerds

"I'm particularly interested in introductions for non-experts to subjects like biology, physics and astronomy, but I thought that opening up the question as broadly as possible would make it most interesting to me and other readers, especially as a future reference-point. I am thinking of books like "Mathematics for the Million", which made math accessible to a great deal of people."